The present invention relates to a gamma correction circuit using an analog multiplier, and more particularly, to a gamma correction circuit for overcoming electro-optic nonlinearity associated with liquid crystal material to maintain a linear output while driving a liquid crystal display panel.
Recently, improved LCD picture quality has been required to take advantage of advanced video signal input/output technology and a variety of new video media. Suitable liquid crystal materials for these applications exhibit a gray scale over a range of applied voltages. That is, for a given liquid crystal material, transmittance increases generally with increasing voltage. The relationship between applied voltage and transmittance, however, is not linear. For example, as shown in FIG. 1, which is a typical electrooptic characteristic of an LCD, transmittance as a function of applied voltage is not linear around V.sub.10 and V.sub.90. The curve shown in FIG. 1 is commonly referred to as the "gamma", and circuitry used to compensate for these nonlinearities is known as "gamma correction" circuitry.
Gamma correction circuits linearly correct the transmissivity characteristic of a liquid crystal over predetermined nonlinear portion of the characteristic. Typically the gamma correction circuit increases the amplification rate over the nonlinear portions in a chrominance demodulation circuit.
Conventional gamma correction circuits, however, are limited to correcting in either a digital or analog mode. They are also very complicated to make and they frequently generate an unstable video signal output.
FIG. 2 is a block diagram of an LCD driver having the conventional gamma correction circuit, which controls the amplification rate of the nonlinear portions in the chrominance demodulation block when a video signal is applied to an LCD pixel. The video signal, having a controlled amplification rate, is supplied to a gamma compensator circuit 20, which, in turn, feeds a gamma compensated value to the Y-driver 22 of the LCD array 24. A controller outputs horizontal and vertical sync signals to the X and Y drivers, 22 and 28, respectively. The X and Y drivers then supply appropriate signals to drive the LCD array 24.
FIG. 3 is a block diagram illustrating a conventional digital gamma correction circuit in greater detail. As shown in FIG. 3, the gamma correction circuit receives a video signal and sets a proper gamma value for correction. FIG. 4 is a graph showing the input/output characteristic of the conventional digital gamma correction circuit. The conventional gamma correction techniques will now be discussed with reference to FIGS. 3 and 4.
As seen in FIG. 3, A/D converter 30 receives an analog input video signal and converts it into a digital signal. This digital signal is detected in a data detector 31. A gamma setting circuit 32 establishes a gamma value for the detected signal by retrieving a predetermined gamma value from a ROM look-up table (not shown). The output of gamma setting circuit 32 is then supplied to an adder 33, which sums the signal output from A/D converter 30 and the signal output from gamma setting circuit 32. The summed result is output to a D/A converter 34, which converts the digital signal output from adder 33 into analog, and feeds the converted signal to a TFT pixel. Through this process, gamma correction is performed.
In FIG. 4, correction for an arbitrary point P is carried out as follows. When a data input voltage D.sub.VP at point P is output without gamma correction, the corresponding transmissivity of the input voltage becomes L.sub.P ". This causes the transmissivity to be nonlinear over the entire range of input data voltages. However, when input data voltage D.sub.VP ' at point P' is applied to the pixel, the transmissivity becomes L.sub.P, which is at a linear portion of the transmissivity characteristic curve.
Accordingly, the gamma set value becomes D.sub.VP -D.sub.VP ' because value D.sub.VP ' must be input instead of value D.sub.VP. The gamma set value varies with respect to the respective input data voltages of the nonlinear portions. The section from point A to point B in the graph shown in FIG. 4 has a linear transmissivity, requiring no gamma correction. Thus, when input data voltage D.sub.VQ ' at point Q' is applied to the pixel, transmissivity becomes L.sub.Q " and the characteristic becomes linear. If, however, the input voltage is Q, the output voltage is D.sub.VQ ' which is at a nonlinear portion of the transmissivity characteristic curve.
As discussed above, the digital gamma correction circuit performs D/A or A/D conversion with respect to the analog data signal. In converting the analog data into digital or the digital data into analog, round-off errors occur and data is frequently not expressed accurately. As a result, the number of gray scales or gradations which can be gamma corrected is limited.
FIG. 5 is a block diagram of a conventional analog gamma correction circuit, which forms a gamma-corrected output using several differential amplifiers. FIG. 6 is a graph showing the input/output characteristic of this gamma correction circuit. The conventional analog gamma correction will now be explained below with reference to FIGS. 5 and 6.
As seen in FIG. 5, three reference voltages V.sub.RL, V.sub.RM and V.sub.RH are applied to the respective differential amplifiers 505, 515 and 525, each of which having different gains and operating within different voltage ranges. Because the output current of each respective differential amplifier is I.sub.out =GM(Vin-Vout), current i.sub.out flowing through a load resistor R.sub.L becomes i.sub.1 +i.sub.2 +i.sub.3, which is controlled according to the input voltage range. An input/output characteristic of the gamma correction circuit is thus obtained, as shown in FIG. 6. The conventional analog gamma correction circuit shown in FIG. 5, however, uses more than three differential amplifiers, and is thus complicated.
As mentioned above, the digital gamma correction circuit causes round-off error in converting the analog data into digital or converting digital data into analog, thereby limiting the number of gradations for gamma correction. Further, the analog gamma correction circuit uses more than three differential amplifiers, and is therefore a very complicated circuit. Neither the conventional analog nor digital circuits can provide suitable gamma correction of the input signal.